Machine safety in the USA

OSHA regulations

All general and machine-specific safety standards for machines are included in the U.S. Code of Federal Regulations, Title 29, Part 1910, Subpart 0. The following list shows a few examples. Supplementary information can be found at www.osha.gov.

Extract from the U.S. Code of Federal Regulations, Title 29, Part 1910, Subpart 0

There is no uniform federal legislature in the USA that regulates the responsibility of the manufacturer or supplier. Each federal US state, however, is required by OSHA, 1970, Section 18 to develop its own occupational health and safety program. OSHA provides additional information on each of these programs on the www.osha.gov or www.osha-slc.gov websites.

US Standards ANSI, NFPA, UL (National Consensus Standards)

In addition to the OSHA standards, the OSHA authority is authorized to monitor and enforce compliance with National Consensus Standards. These are standards, occupational health and safety regulations or modifications of such, which

• have been adopted and published by a nationally recognized standards-setting organization (e.g. ANSI, UL),
• are recognized by the Secretary of Labor as standards,
• deal as international standards (IEC, ISO) with topics or specialist areas that are not covered by a US standard.

U.S. National Consensus Standards are therefore standards that apply as supplementary to the OSHA standards. The following are some of the bodies that provide such standards:

• American National Standards Institute (ANSI) www.ansi.org
• National Fire Protection Agency (NFPA) www.nfpa.org

Selection of important U.S. National Consensus Standards in the machine safety area:

Strategy for risk reduction

The U.S. Code of Federal Regulations, Title 29, Part 1910, Subpart 0 requires that with the construction of machinery risks must be analyzed and, where required, protective devices must be provided to protect the operator.
Technical Report ANSI B11.TR3:2000 includes proposals for assessing, analyzing and reducing risks on tool-making machines.

OSHA/ANSI provides the following hierarchical procedure for risk reduction:

1. Identification and analysis of the risk (see ANSI B11.TR3:2000)
2. Removal of the risk with constructive measures
3. Reduction of the risk with technical protective devices
4. Warning signals and warning information
5. Personal protective equipment for the operating personnel
6. Training the operating personnel

International standard ISO 12100 "Safety of machinery – General principles for design – Risk assessment and risk reduction" has a similar structure. It provides detailed assistance with the identification of hazards, describes the risks to be considered by the designer, contains design principles and a method for safe construction and risk minimization. It also describes an iterative method for risk analysis, risk assessment and risk reduction for achieving the required machine safety. Existing machine-specific standards, such as type C EN standards, for example, must be considered with priority.

On risk assessment ISO 12100

Control Reliability

OSHA 1910.211
Logically contains the following requirements: A control system must be constructed in such a way that

• a fault that occurs inside the system does not prevent the normal stop process from being activated,
• another machine cycle cannot be executed before the fault has been removed and
• the fault can be revealed by a simple test, or displayed by the control system.

ANSI B11.19-2003
Subpart 3.14 logically defines Control Reliability as follows:

Control Reliability is the capability of the machine control system, the safeguarding, other control components and related interfacing to achieve a safe state in the event of a fault within their safety related functions.
Subpart E.6.1 specifies and limits:
Control Reliability can't prevent the reinitation of a machine cycle in case of a

• severe mechanical failure or
• a simultaneous failure of more components.

The standard provides the following information on the structural setup:
Control Reliability is not guaranteed by simple redundancy. Monitoring must be made to ensure that the redundancy remains effective.

ANSI B11.20
The following is also logically stated with regard to the control system structure in ANSI B11.20, Subpart 6.13:

Protection against the consequences of failure of control components should not depend solely upon simple redundancy. A failure of one component of two or more parallel or serially switched control components can remain unnoticed with simple or unmonitored redundancy. The appearance of a safe operation is maintained. If another element now also fails in another redundant circuit, this can result in a dangerous state. A monitoring of redundant control system structures and the uncovering of and safe reaction to such single errors is therefore mandatory.

ANSI / RIA R15.06-1999
This ANSI standard contains further functional requirements for Control Reliability and also includes statements on errors that have common causes, such as overvoltage. Note: The term "common" means that these causes can have the same, simultaneous effect on the redundantly set up control channels.

• The monitoring must activate a stop signal when a fault is detected.
• A warning must be issued if the hazard continues to exist after the movement has been brought to a stop.
• After the fault has been detected a safe state must be maintained until the fault has been removed.
• Failures with common causes (e.g. overvoltage) must be considered when the probability of occurrence of such failures is high.
• A single fault should be detected at the time at which it occurs. If this is not practical the fault should be detected the next time the safety function is requested.

Comparison of the ANSI, IEC/EN requirements for safety-related controls
There is no precise concurrence on the definition of functional safety or Control reliability in the US and IEC/EN world of standards. The requirements of category 3 of ISO 13849-1 come relatively close to the OSHA/ANSI requirements:

• The safety-related parts of control systems and/or their protective devices and their components must be designed, constructed, selected and combined in accordance with the applicable standards in such a way that they can withstand the expected influences and effects.
• Proven-in-practice safety principles must be applied in design and construction. Safety-related parts must be designed so that:
• A single fault in each of these parts does not cause the loss of the safety function.
• The single faults are detected whenever this is reasonably possibly.

The behavior when a fault of a safety-related control unit in accordance with category 3 occurs is specified as follows:

• If a single fault occurs, the safety function is always maintained.
• Some but not all faults are detected.*
• An accumulation of undetected errors can lead to the loss of the safety function.*

*) The risk assessment shows whether or not the complete or partial loss of the safety function(s) that the faults cause is manageable.

Note
The SISTEMA PC software of the German Institute for Occupational Safety and Health (IFA) is used for the calculation and evaluation of the functional safety of control systems in accordance with ISO 13849-1. It is available as freeware under www.dguv.de/ifa/praxishilfen.